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What the pandemic can teach us about ways to reduce air pollution

The COVID-19 pandemic was not just a shock to the human immune system. It was also a shock to the terrestrial system that drastically changed air quality in cities around the world.

When countries around the world struggled to contain the disease, they imposed temporary closures. Scientists are now examining data collected by satellite and on the ground to understand what this break in human activities can tell us about the atmospheric cocktail that pollutes the city. Many of these preliminary data were shared at the annual meeting of the American Geophysical Union in December.

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It was already known that the activities of the peoples were reduced enough to produce a dramatic drop in greenhouse gas emissions in April, as well as a drop in human-produced seismic noise (SN: 5/19/20; SN: 7 / 23/20). However, that period of calm did not last and carbon dioxide emissions began to rise towards the summer. April 2020 saw a 17 percent drop in global fossil fuel monthly CO2 emissions, but by the end of the year annual CO2 emissions to the globe were only 7 percent lower than in 2019. That reduction was too short. , compared to the hundreds of years that gas can persist in the Earth’s atmosphere to put a gap in the planet’s atmospheric CO2 level (SN: 8/7/20).

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But in addition to briefly reducing emissions of global warming gases, this abrupt halt in many human activities, especially commuter traffic, has also created an unprecedented experiment for scientists to examine the complicated chemistry of air pollutants in cities. By altering the usual mix of pollutants gliding over cities, strikes can help scientists better understand another long-standing misery for human health: poor air quality in many cities.

That’s not to say the pandemic has silver lining, says Jessica Gilman, tropospheric chemist at the National Oceanic and Atmospheric Administration in Boulder, Colorado. "Poverty is not a solution to our global environmental challenges."

But there is now a wealth of data from cities around the world on how the pandemic has altered regional or local concentrations of ozone precursors, a major component of smog. These precursors include nitrogen oxides and volatile organic compounds, produced by traffic, as well as methane, produced by the oil and gas industry. With satellites, scientists can also assess how levels of these pollutants have changed around the world.

Building a global picture of changing pollution in cities is no easy task. Researchers find that the impact of the pandemic on the levels of various pollutants has been very regional, affected by differences in wind and rain, as well as photochemical interactions with sunlight, the intensity of which also changes with the season.

That strong variety of regional effects was evident, for example, in the different post-pandemic ozone levels in Denver and New York. Nitrogen oxide gases produced by traffic are a powerful precursor to high levels of ozone in cities, which can damage the lungs and cause respiratory diseases. The United States has made progress in reducing these gases in recent decades, but there has been no corresponding drop in ozone levels, Dan Jaffe, an environmental chemist at Bothell University of Washington, told the Dec. 9 meeting.

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The closures gave investigators an idea of ​​why, Jaffe says. From March 15 to July 23, New York City had a 21% decrease in nitrogen dioxide, one of several nitrogen oxide gases, compared to 2019 levels. Although shutdowns were stricter during the In the spring months, it was found that reductions in nitrogen dioxide during the summer were more closely linked to changes in the city's ozone levels, the researchers found. “We are seeing a sharp reduction in summer ozone this year,” Jaffe said at the meeting, citing unprecedented data.

This is because in the summer months, heat and sunlight react with the precursor gases in the atmosphere, such as nitrogen dioxide, creating a toxic cocktail. This kind of knowledge may be a blessing for policymakers in a non-pandemic year, suggesting nitrogen oxide regulations should focus more on the summer, Jaffe says. "It's good evidence that the NOx reductions that will extend through July 2020 have had a significant impact."

In Denver, however, ozone did not fall as consistently, possibly because forest fires were beginning to accelerate across the western United States in late summer (SN: 21/12/20). Fires produce nitrogen oxides, carbon monoxide, and fine particles that can also help increase ozone in the soil.

“There are different patterns in different cities,” Jaffe says. "There are a lot of factors to solve and a lot of work to do." Armed with a wealth of new 2020 data, scientists hope to be able to move forward.

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